The feeds are dynamically generated so they do not have a "file" in the traditional way.
As for your feeds going 404, have you changed your permalinks settings lately? Go to the permalinks settings in WordPress and re-save the permalinks settings. That might fix it. (likely not but worth a try).
Another thing is to make sure PowerPress is enabled. (I know, but you never know).
Looks like you have a category of page called "podcast". That can also interfere with podcast feeds in PowerPress.
If none of the above fix it, I would turn off ALL plugins other than PowerPress and see if the feeds come back. If not, change your theme to one of the stock WordPress themes and see. It's very likely that some other plugin or your theme is taking out the RSS feeds.
-Mike